It would theoretically be possible if your paladin somehow lost his LG status, but I don't think either paladin falling or alignment changes were implemented in IWD2 (at least not without mods). If you use a character editor you could do it, either by manually changing the class or by changing the alignment and then multiclassing normally inside the game. It's kind of a weird combination, though, as it would essentially mean that your character was abandoning his deity and quest for goodness in exchange for BEING ANGRY ALL THE TIME.